1. Field of the Invention
This invention relates generally to downhole packers and, more particularly, to inflatable packers for sealing an annular space in a well bore. Specifically, the present invention relates to an improved gripping mechanism for maintaining such packers in position within the well bore.
2. Description of the Prior Art
An inflatable packer is a down hole tool which is inflatable with a fluid or with some other mechanism to seal off an annular space in a well bore. Such well bores are, at times, left unlined so that the well bore wall is the material bored into. At other times, the well bore is lined with a metal casing; and in such cases, the well bore wall is the casing. Although not so limited, the packer of the invention is particularly suited for isolating zones within a well for such purposes as cementing, fracturing, treating, testing, preventing gas migration to the surface, and for gravel pack operation.
Early prior art inflatable packers relied upon the inflation and resulting expansion of an elastomeric bladder to both seal off an annular space in the well bore as well as to provide frictional force to prevent axial displacement of the packer within the well bore while it is deployed. In such an arrangement, the amount of frictional force generated is limited to the frictional properties of the materials of the bladder and the well bore wall, multiplied by the force per unit area (or pressure density) supplied by the pressurized fluid within the bladder. A recognized problem with prior art packers has been the inability of the inflatable packers to engage the well bore wall surface with adequate frictional force to resist external forces placed upon the packer, which tend to displace the packer axially from its deployed position.
Attempts to solve this displacement problem, by increasing the frictional force, have included a variety of different mechanisms. One such attempt places woven or calendered steel cable around the outer surface of the bladder, so that the cable is pressed against the well bore wall surface when the bladder is inflated. This is disclosed in U.S. Pat. No. 4,424,861 to Carter, et al. Another arrangement includes placing multi-edged particles in two bands around the outer surface of the bladder that are located near the ends of the bladder and thereby leaving the middle region of the outer surface bare of such particles, so as to allow this middle region to provide the necessary sealing engagement. In still another arrangement, inverted cone grippers (where the annular edge of the cone's base is intended to engage the well bore wall surface) are placed around the outer surface in the same manner as the multi-edge particles discussed above. Another arrangement places serrated plates (where the edges of the serration are intended to engage the well bore wall surface) around the outer surface in a similar manner. Examples of these last three arrangements are illustrated in Canadian Patent No. 702,327 and U.S. Pat. No. 3,035,639, Brown et al. In still another prior art device disclosed in U.S. Pat. No. 2,970,651 to Roberts, overlapping steel bands, with spikes distributed over every second band, are placed covering the entire outer surface. All of these attempts at solving the problem of displacement of inflatable packers have been apparently inadequate by either not providing adequate frictional force or not providing an effective seal.
The gripping device of U.S. Pat. No. 4,424,861 apparently lacks the ability to adequately press the gripping surface into the well bore wall to cause embedment of the same and thus not creating the required frictional force. It has been discovered that this is because the geometry of the gripper device generates little or no multiplication of pressure density imparted from the bladder to the surface of the gripping device that contacts the well bore wall surface. Further, the rounded edgewise contact made with the wall by the gripping device is not conducive to creating adequate frictional force. The gripping device also interferes with the sealing engagement of the outer surface of the bladder. The gripping devices of Canadian Patent No. 702,327 also have inherently limited ability to multiply the pressure density, as they rely upon edgewise contact between the gripping devices and the well bore wall surface.
Further, it has been determined that by distributing the gripping devices and the devices of U.S. Pat. No. 4,424,861 near the ends of the bladder, the well fluid has the opportunity to apply a lifting force on the bladder immediately around the gripping devices, thereby reducing the pressure such devices apply to the well bore wall as well as allowing continuous lubrication of the engagement of the gripping device with the well bore wall surface. Both of these effects tend to reduce the frictional force generated by the gripping devices. Thus, there is still a need for inflatable packers which do not exhibit undesirable axial movement within the well bore when acted upon by normal operating well bore fluid forces.